The present invention relates to an anti-lock control device for use in an air-over hydraulic braking system.
Anti-lock control devices for use in air-over hydraulic braking systems are disclosed, for example, in the specification of United States Patent Appln. Ser. No. 07/317,846 now U.S. Pat. No. 4,889,395 and in Japanese Patent Examined Publication No. 1824/1976.
Detailed description will be given of the above-mentioned known devices, with specific reference to FIGS. 5 and 6,
Referring first to FIG. 5, a known braking system has a brake valve 2 connected to a pneumatic pressure source 1. The control device also has an air master cylinder 3 which is equipped with a pneumatic chamber 3a connected to the brake valve 2 and also with a hydraulic pressure chamber 3d which is connected through a modulator 4 to wheel cylinders 5 only one of which is shown. The modulator 4 has a pneumatic chamber 4a which is connected to the brake valve 2 through a normally-opened hold valve 7 which serves as a pneumatic pressure control valve. The pneumatic chamber 4a of the modulator 4 also is communicated with the atmosphere through a normally-closed decay valve 8 which also serves as a pneumatic pressure control valve.
In normal braking operation, as the brake valve 2 is opened, the pneumatic pressure which has been applied to the pneumatic chamber 3a of the air master cylinder 3 is converted into hydraulic pressure which is applied to wheel cylinders 5 through the modulator 4 so as to brake the wheels. When locking of wheel is going to occur during the braking, the hold valve 7 is closed while the decay valve 8 is opened so that the pneumatic pressure in the pneumatic chamber 4a of the modulator 4 is reduced so that the pneumatic piston 4b in the modulator is moved. In consequence, the hydraulic piston 4c connected to the pneumatic piston 4b is moved so as to close the control valve 10 provided in the fluid passage in the modulator 4 while increasing the volume of the hydraulic pressure chamber 4d so that the brake fluid pressure in the wheel cylinders is relieved into the hydraulic pressure chamber 4d so that the braking pressure is reduced to prevent the wheels from being locked. When the wheel lock is avoided, the decay valve 8 is closed and the hold valve 7 is opened so that the pneumatic pressure from the pneumatic source 1 is introduced into the pneumatic pressure chamber of the modulator 4 so that the pneumatic piston 4b and, hence, the hydraulic piston 4c are moved to reduce the volume in the hydraulic pressure chamber 4d. In consequence, the braking hydraulic pressure is recovered and applied to the wheel cylinders so as to brake the wheels.
Referring now to FIG. 6, another known braking system has an air master cylinder 3 provided with a pneumatic pressure chamber 3a which is connected to a pneumatic pressure source 1 through a brake valve 2 and a normally-opened hold valve 7 which serves as the pneumatic pressure control valve. The air master cylinder 3 also has a hydraulic pressure chamber 3d connected to wheel cylinders 5. The pneumatic pressure chamber 3a of the air master cylinder 3 is opened to atmosphere through a normally-closed decay valve 8 which functions as a pneumatic pressure control valve.
In normal braking operation, the brake valve 2 is opened so that the pneumatic pressure applied to the pneumatic pressure chamber 3a of the air master cylinder 3 is converted into hydraulic pressure which is applied to wheel cylinders 5 thereby producing braking effect. When anti-lock control is necessary, the hold valve 7 is closed while the decay valve 8 is opened so that the pressure in the pneumatic pressure chamber 3a of the air master cylinder 3 is reduced, with the result that a pneumatic piston 3b in the air master cylinder 3 is moved. In consequence, a hydraulic piston 3c connected to the pneumatic piston 3b is moved so as to increase the volume of the hydraulic pressure chamber 3d so that the braking hydraulic pressure is relieved into the hydraulic pressure chamber 3d whereby the braking effort is reduced to avoid locking of wheels. When wheel lock is avoided, the decay valve 8 is closed while the hold valve 7 is opened so that the air pressure from the pneumatic pressures source is introduced into the pneumatic pressure chamber 3a of the air master cylinder 3 so that the pneumatic piston 3b and, hence, the hydraulic piston 3c are moved so as to decrease the volume of the hydraulic pressure chamber 3d, so that the braking hydraulic pressure is recovered and applied to the wheel cylinders 5 thereby to put the brake again into effect.
In these known systems, the recovery of the hydraulic braking pressure after an anti-lock control operation, i.e., the rate at which the hydraulic braking pressure rises to produce the braking effect again, is the same as that in the ordinary braking operation.
In general, however, anti-lock control operation is effected when the wheels are rolling on a slippery road, i.e., when the vehicle is running on a road the surface of which is liable to cause a lock of wheels. From this point of view, the recovery of the braking hydraulic pressure need not be done at the same rate as the building up of the braking hydraulic pressure in ordinary braking operation. Rather, the recovery of the braking hydraulic pressure after an anti-lock braking operation is preferably conducted with a smaller rate.